The use of disposable pipette tips with handheld pipettes and automated liquid handling systems is well known. Disposable pipette tips enable repeated use of such pipetting systems to transfer different fluids or different fluid samples without carryover contamination. Disposable pipette tips are normally formed of a plastic material, such as polypropylene, and have a hollow, elongated, generally conical shape. The upper end of the pipette tip typically includes a collar that is mounted to the tip mounting shaft on the pipette device. The mounting shaft includes an internal bore through which air is displaced in order to aspirate liquid sample into and dispense liquid sample from the pipette tip. The far end of the pipette tip has a small opening through which liquid sample is received into and dispensed from the barrel of the pipette tip.
Disposable pipette tips have historically relied on tapered fits between the mounting shaft and the pipette tip collar, as well as sealing rings on the inside circumference of the pipette tip collar, to secure and seal the pipette tips to the mounting shaft. In most cases, the fit between the mounting shaft and the disposable tip is achieved by pushing the tapered mounting shaft into the tapered pipette tip collar until it wedges into the tip. At this point, a seal is achieved between the tip collar and the mounting shaft as a result of crushing the sealing ring and/or stretching the diameter of the collar. In addition to achieving a proper seal, it is also important that position and orientation of the mounted tip also be stable in the face of lateral momentum or slight knocking forces that are typical during normal use such as during touch-off on the sidewall of a vessel. In order to assure tip stability, users tend to jam the pipette mounting shaft into the tip with excessive force.
Various systems have been devised to provide proper sealing and stability without requiring excessive mounting and ejection forces. For example, the use of cylindrical mounting shafts and cylindrical tip collars lessens mounting and ejection forces. Also, it is well known to use a step within the pipette tip collar as a depth limiting means for the pipette mounting shaft. Even so, such systems typically require the force of an interference fit or stretching of the pipette tip collar to maintain stable engagement of the pipette tip and ensure a reliable seal of the collar against the mounting shaft.
A further approach is described in U.S. Patent Application Publication No. US 2005/0175511 A1 in which the pipette tip collar has inwardly projecting, cantilevered fingers that latch over a circumferential rim on the mounting shaft. In this approach, sealing is achieved by an O-ring on the mounting shaft that is located below the location of the latching engagement. Ejection of the tip is achieved by modifying the ejection mechanism on the pipette so that it can release the inwardly projecting fingers on the pipette tip before asserting pressure to eject the tip from the mounting shaft.